Electromagnetic wave transmissive metal walls utilizing dielectric rods



Aug. 1, 1%? H, A, HEELER 3,334,349

' ELECTROMAGNETIC E TRANSMISSIVE METAL WALLS UTILIZING DIELECTRIC RODS Filed Dec. 30, I963 2 Sheets-Sheet 1 FIG. 1

1967 H. A. WHEEL 3,334,349

ELECTROMAGNETIC WAVE TRAN VE TAL WALLS UTILIZING DIELECTR R0 6 Filed Dec. 5O, 19 3 2 Sheets-Sheet 2 United States Patent G ELECTROMAGNETIC WAVE TRANSMISSIVE WALLS UTILIZING DIELECTRIC Harold A. Wheeler, Great Neck, N.Y., assignor to Hazeltine Research, Inc, a corporation of Illinois Filed Dec. 30, 1963, Ser. No. 334,541 Claims. (Cl. 343-872) This invention relates to walls for providing protection from extreme environmental stresses, while transmitting electromagnetic waves.

A microwave antenna may require protection from environmental factors such as wind, over-pressure, precipitation and extremes of temperature. This protection can be furnished by an enclosure which must be essentially transparent to the electromagnetic waves radiated by the antenna. One form of typical prior art enclosures are radomes made of a wall of dielectric material, such as fibre glass, of either a solid or layered cross-section. However, in applications having a very severe environment, especially where very high over-pressures may occur, a stronger and more durable structure is required to protect an antenna.

An object of this invention therefore is to provide new and improved walls capable of providing protection from severe environmental stresses while transmitting electromagnetic waves substantially without distortion.

An additional object is to provide such walls utilizing a metal shell whose thickness is chosen on the basis of the stresses to be encountered and the electromagnetic Wave transmission characteristics of the wall are substantially independent of the shell thickness.

In accordance with the present invention a wall, for resisting environmental stresses while transmitting electromagnetic waves, comprises a metal shell having an array of holes of circular cross-section, with the thickness of the shell being greater than the diameter of the holes and also greater than the minimum center-to-center spacing of the holes and a plurality of dielectric rods supported in the holes and forming in combination therewith a plurality of dielectric filled waveguides, each rod being arranged to provide sections of different characteristic impedance at each end of each of the waveguides for providing impedance matching independently at both ends of the shell between free space and the dielectric filled waveguides formed by the combination of the holes and the rods. The wall is so constructed and arranged as to provide protection from extreme environmental stresses while transmitting electromagnetic waves substantially without distortion.

For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings, and its scope will be pointed out in the appended claims.

In the drawings:

FIG. 1 shows a general view of a complete radome utilizing a wall constructed in accordance with the invention;

FIGS. 2 and 3 are views of a section of the wall of the FIG. 1 radome showing greater detail; and

FIGS. 4 and 5 show portions of different forms of walls constructed in accordance with the invention.

Referring to FIGS. 1, 2 and 3, there is shown one form of wall constructed in accordance with the invention, more particularly there is shown a radome for encasing a microwave antenna. FIGS. 2 and 3 are enlarged views of the portion of the wall of the FIG. 1 radome within the dotted rectangle. As shown, the wall includes a metal shell having an array of holes. The shell 10 may be 3,334,349 Patented Aug. 1, 1967 constructed of steel, aluminum or other desired metal. The wall, as shown, also includes a plurality of dielectric rods 12 which are supported in the holes in the metal shell 10. The rods 12 are shaped to provide impedance matching at both sides of the shell 10 between free space and the dielectric-filled waveguides formed by the combination of the holes and the rods. As shown in FIG. 3, the rods 12 go through the shell 10 and each end of each rod is substantially flush with the surface of the shell 10. It is considered particularly desirable to construct the rods 12 of quartz where great extremes of heat are to be encountered, but any suitable dielectric may be used. It will be understood that the FIG. 1 radome is completely covered by a regular array of rods 12 supported in holes, even though only a small area of rods is actually shown. Also, the FIG. 3 view should exhibit a small amount of curvature, however, if the radome of FIG. 1 were very large in diameter the small area of the wall shown in FIGS. 2' and 3 would be essentially fiat as illustrated.

In operation, the combination of the holes and the dielectric rods 12 form a plurality of dielectric-filled waveguides passing through the metal shell 10. A wave originating from the left and incident on the section of the wall shown in FIG. 3 is captured by the waveguide openings and then propagates through the dielectric-filled waveguides and finally is re-radiated by the right-hand end of the waveguides and the wave then continues to the right. If the rods are spaced closely enough in comparison to a wavelength, the uniformity of the incident wave is essentially maintained in the re-radiated wave. It will be understood that the wall is a reciprocal device and works similarly for waves incident from either side. The waveguides are circular in cross section so as to operate with incident waves of any linear polarization or circular polarization. The dielectric loading of the waveguides allows the waveguides to be small enough to permit them to be closely spaced relative to the operating wavelength. If the rod diameter, dielectric constant, and construction are properly chosen there will be very little reflection at either side. Therefore, nearly all of an incident electromagnetic wave will be transmitted through the wall substantially without distortion.

A practical wall will normally be required to operate over a band of frequencies. In this case, the average operating wavelength can be used as an approximation in the design of the wall. Design of walls in accordance with the invention can be carried out using established principles of antenna and waveguide design once the'basic concepts of the invention are understood.

Referring now to FIG. 4, in a view similar to the view of FIG. 3, there is shown a portion of a different type of wall constructed in accordance with the invention. In FIG. 4 each hole through the shell 10 is in the form of a simple circular hole with a comically-flared end region at each end. Each hole is substantially totally filled by a dielectric rod 14 so that each hole becomes effectively a dielectric-filled circular waveguide with flared horn-like end regions which provide impedance matching. The rods 14 may be formed by pouring a liquified dielectric into the holes and allowing the dielectric to harden, or by inserting two similar dielectric pieces, one from either side of the shell, so that the ends butt together so as to effectively form a continuous dielectric rod through the shell 10, for example.

Referring now to FIG. 5 there is shown a portion of another type of wall constructed in accordance with the invention. In FIG. 5 the shell 10 has a plurality of simple circular holes formed through it. In each hole there is inserted a dielectric rod 16 which has the form of a right circular cylinder with a circumferential groove 18 near each end for providing impedance matching. Included in FIG. 5 are actual dimensions, in wavelengths, of a portion of a wall which was actually built and tested, and found to provide excellent transmission characteristics over a band of frequencies. The portion of the shell was constructed of steel and the rods 16 were of quartz (fused silica). The rods were arranged in a square-type array as shown in FIG. 2.

The walls of FIGS. 3, 4 and 5 have in common certain physical attributes which lend themselves to the construction of walls able to withstand extreme stresses. First, in each of these walls the thickness of the shell 16 is greater than the minimum center-to-center spacing of the holes in which the dielectric rods are supported. The minimum center-to-center spacing is the spacing measured relative to horizontally or vertically adjacent holes in FIG. 2; as differentiated from the spacing between diagonally adjacent holes in FIG. 2. Second, the thickness of the shell 10 is greater than the diameter of the holes in FIGS. 3, 4 and 5. In addition, the size and spacing of the holes are such that the volume of metal exceeds the volume represented by the holes. By considering a unit section, such as enclosed by dotted rectangle 20 in FIG. 2, it will be obvious that the volume of metal included exceeds the volume represented by the portions of the holes included. Walls constructed in accordance with the invention may typically include holes with a minimum center-to-center spacing of from /2 to 1 times the operating wavelength and a hole diameter of approximately /2, times said spacmg.

It should be understood that in walls constructed in accordance with the present invention, each side of the wall is independently designed to pass a desired wave and while inside the shell 10 waves simply propagate along a dielectric-filled circular waveguide. Since this is true, there is no inherent limitation on the thickness of the metal shell 10 of the wall. In FIG. 5, the metal shell is shown as being 3.04 wavelengths thick but this is not a critical dimension. Thus, the metal shell 10 could be constructed of steel many inches thick, if desired, and an extremely strong structure would be obtained. Since the dielectric rods 12 are not relied on for strength, the dielectric material can be chosen to withstand other factors of the environment. If quartz (fused silica) is used, the wall would be resistant to extremely high temperatures as well as other environmental factors.

Compared with other designs capable of similar performance, the invention as described above facilitates economical construction. The dielectric rods can easily be inserted in the holes in the metal shell 10' and then cemented or otherwise held in place. Each rod is simple in form and identical to the others, so that mass production of the rods is readily possible. A wide variety of rod spacings, and cross sections can be utilized in different applications.

While there have been described what are at present considered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention and it is, therefore, aimed to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A wall, for encasing a microwave antenna, comprising:

a metal shell having a predetermined regular array of circular holes;

and a plurality of substantially identical solid dielectric rods supported in said holes so that each end of each rod is substantially flush with the surface of said shell, each rod having the form of a right circular cylinder with a circumferential groove near each end for impedance matching purposes;

whereby an antenna can be protected from extreme 4 environmental stresses while electromagnetic waves are permitted to pass between such antenna and points external to the wall substantially without distortion.

2. A wall for encasing a microwave antenna comprising:

a metal shell having a predetermined regular array of substantially identical holes, each hole having the form of a simple circular hole with conically flared end regions;

and a plurality of solid dielectric rods substantially totally filling each of said holes so that hole becomes effectively a dielectric-filled circular waveguide with flared horn-like end regions for impedance matching purposes;

whereby an antenna can be protected from extreme environmental stresses while electromagnetic waves are permitted to pass between such antenna and points external to the wall substantially without distortion.

3. A wall, for resisting environmental stresses while transmitting electromagnetic waves, comprising:

a metal shell having an array of holes of circular crosssection, with the thickness of the shell being greater than the diameter of said holes and also greater than the minimum centerto-center spacing of said holes;

and a plurality of dielectric rods supported in said holes and forming in combination therewith a plurality of dielectric filled waveguides, each rod being arranged to provide sections of different characteristic impedance at each end of each of said waveguides for providing impedance matching independently at both ends of said shell between free space and the dielectric filled waveguides formed by the combination of said holes and said rods;

the wall being so constructed and arranged as to provide protection from extreme environmental stresses while transmitting electromagnetic waves substantially without distortion.

4. A wall, for resisting environmental stresses while transmitting electromagnetic waves, comprising:

a metal shell having a regular array of holes of circular cross-section in which said holes have a minimum center-to-center spacing in the range of from onehalf to one times the operating wavelength and each of said holes has an identical diameter of approximately two-thirds times said spacing;

and a plurality of dielectric rods supported in said holes and forming in combination therewith a plurality of dielectric filled waveguides with the ends of each rod being substantially flush with the surfaces of said shell, each rod arranged to provide sections of different characteristic impedance at each end of each of said waveguides for providing impedance matching independently at both ends of said shell between free space and the dielectric filled waveguides formed by the combination of said holes and said rods;

the wall being so constructed and arranged as to provide protection from extreme environmental stresses while transmitting electromagnetic waves substantially without distortion.

5. A wall in accordance with claim 4 wherein the thickness of the shell is greater than the minimum centerto-center spacing of said holes.

References Cited UNITED STATES PATENTS 2,415,089 2/1947 Feldman 343785 X 2,607,009 8/1952 Aftel. 2,636,125 4/1953 Southworth 343-909 X ELI LIEBERMAN, Primary Examiner.

P. L. GENSLER, Assistant Examiner. 

3. A WALL, FOR RESISTING ENVIRONMENTAL STRESSES WHILE TRANSMITTING ELECTROMAGNETIC WAVES, COMPRISING: A METAL SHELL HAVING AN ARRAY OF HOLES OF CIRCULAR CROSSSECTION, WITH THE THICKNESS OF THE SHELL BEING GREATER THAN THE DIAMETER OF SAID HOLES AND ALSO GREATER THAN THE MINIMUM CENTER-TO-CENTER SPACING OF SAID HOLES; AND A PLURALITY OF DIELECTRIC RODS SUPPORTED IN SAID HOLES AND FORMING IN COMBINATION THEREWITH A PLURALITY OF DIELECTRIC FILLED WAVESGUIDES, EACH ROD BEING ARRANGED TO PROVIDE SECTIONS OF DIFFERENT CHARACTERISTIC IMPEDANCE AT EACH END OF EACH OF SAID WAVESGUIDES FOR PROVIDING IMPEDANCE MATCHING INDEPENDENTLY AT BOTH ENDS OF SAID SHELL BETWEEN FREE SPACE AND THE DIELECTRIC FILLED WAVEGUIDES FORMED BY THE COMBINATION OF SAID HOLES AND SAID RODS; THE WALL BEING SO CONSTRUCTED AND ARRANGED AS TO PROVIDE PROTECTION FROM EXTREME ENVIRONMENTAL STRESSES WHILE TRANSMITTING ELECTROMAGNETIC WAVES SUBSTANTIALLY WITHOUT DISTORTION. 